【mq读书笔记】mq索引文件刷盘

索引文件的刷盘并不是采取定时刷盘机制，而是每更新一次索引文件就会将上一次的改动刷写到磁盘。

同步刷盘：

GroupCommitRequest将被提交到GroupCommitService线程，GroupCommitService线程处理GroupCommitRequest对象后将调用wakeupCustomer方法将消费发送线程唤醒。并将刷盘告知GroupCommitRequest。

这里将写操作和读操作做了分离，避免了任务提交与任务执行的锁冲突

GroupCommitService每处理一批同步刷盘请求后休息10ms，然后继续处理下一批，其任务的核心实现为doCommit方法：

CommitLog.this.mappedFileQueue.flush最终调用MappedByteBuffer#force()方法。

处理完所有同步刷盘任务后，更新刷盘检查点StoreCheckPoint中的physicMsgTimestamp，但是并没有执行检查点的刷盘操作，检查点的刷盘操作将在刷写消息队列文件时触发。

异步刷盘

CommitRealTimeService线程默认每200ms将ByteBuffer新追加的内容的数据提交到MappedByteBuffer中:

FlushRealTimeService线程默认每500ms将MappedByteBuffer中新最佳的内存(wrotePosition减去上一次刷写位置flushedPositiont)通过调用MappedByteBuffer#force()方法将数据刷写到磁盘:

class FlushRealTimeService extends FlushCommitLogService {
        private long lastFlushTimestamp = 0;
        private long printTimes = 0;
 
        public void run() {
            CommitLog.log.info(this.getServiceName() + " service started");
 
            while (!this.isStopped()) {
                boolean flushCommitLogTimed = CommitLog.this.defaultMessageStore.getMessageStoreConfig().isFlushCommitLogTimed();
 
                int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushIntervalCommitLog();
                int flushPhysicQueueLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogLeastPages();
 
                int flushPhysicQueueThoroughInterval =
                    CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogThoroughInterval();
 
                boolean printFlushProgress = false;
 
                // Print flush progress
                long currentTimeMillis = System.currentTimeMillis();
                if (currentTimeMillis >= (this.lastFlushTimestamp + flushPhysicQueueThoroughInterval)) {
                    this.lastFlushTimestamp = currentTimeMillis;
                    flushPhysicQueueLeastPages = 0;
                    printFlushProgress = (printTimes++ % 10) == 0;
                }
 
                try {
                    if (flushCommitLogTimed) {
                        Thread.sleep(interval);
                    } else {
                        this.waitForRunning(interval);
                    }
 
                    if (printFlushProgress) {
                        this.printFlushProgress();
                    }
 
                    long begin = System.currentTimeMillis();
                    CommitLog.this.mappedFileQueue.flush(flushPhysicQueueLeastPages);
                    long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp();
                    if (storeTimestamp > 0) {
                        CommitLog.this.defaultMessageStore.getStoreCheckpoint().setPhysicMsgTimestamp(storeTimestamp);
                    }
                    long past = System.currentTimeMillis() - begin;
                    if (past > 500) {
                        log.info("Flush data to disk costs {} ms", past);
                    }
                } catch (Throwable e) {
                    CommitLog.log.warn(this.getServiceName() + " service has exception. ", e);
                    this.printFlushProgress();
                }
            }
 
            // Normal shutdown, to ensure that all the flush before exit
            boolean result = false;
            for (int i = 0; i < RETRY_TIMES_OVER && !result; i++) {
                result = CommitLog.this.mappedFileQueue.flush(0);
                CommitLog.log.info(this.getServiceName() + " service shutdown, retry " + (i + 1) + " times " + (result ? "OK" : "Not OK"));
            }
 
            this.printFlushProgress();
 
            CommitLog.log.info(this.getServiceName() + " service end");
        }